There are a number of existing techniques by which images of the inside of a borehole are obtained. However, most of these rely on reconstructing an image on the basis of measurements made of other than optical properties. Measurements used for such imaging include electrical conductivity or permittivity, acoustic impedance or slowness, or density of photoelectric effect. It is also desirable to obtain optical images to better understand the formation and the state of the borehole.
Certain tools have been proposed for obtaining optical images in boreholes. Several have a camera pointed axially down the borehole and can image flowing fluids together with some neighbouring part of the borehole wall. Such tools are dependent on the well fluid being either substantially clear, or the well being shut in and the well fluid being replaced with a clear fluid such as water. An example of such a tool can be found in WO 0206631 A (DHV INTERNATIONAL INC) 24 Jan. 2002.
Proposals for providing images of the borehole wall with an axially aligned camera include the use of hemispherical or conical mirrors coaxial with the camera, see for example EP 0264511 A (SOCIETE DE PROSPECTION ELECTRIQUE SCHLUMBERGER) 27 Apr. 1988, EP 0846840 A (SCHLUMBERGER LTD ET AL) 10 Jun. 1998 or U.S. Pat. No. 2,912,495 (JAMES MOON ET AL) 10 Nov. 1959. The use of fish-eye lenses is also proposed to improve the radial coverage of the image.
An alternative to this can be found in U.S. Pat. No. 5,134,471 (NORANDA INC) 28 Jul. 1992 which describes the use of a radially directed camera which is rotated around the tool axis to provide full coverage of the borehole wall.
The use of hemispherical or conical images or fish-eye lenses suffers from the problems of distortion of images leading to the need to have complex compensating camera optics to obtain a useful image. Furthermore, the limited space available in most boreholes means that it is often not possible to mount the camera radially. Also, the mechanism to rotate the whole camera can be complex.
It is one object of the invention to provide a tool using a camera to obtain images of a borehole wall that does not suffer from the image distortion problems or complexity of the prior art.